LCDs are commonly used as display devices for compact electronic apparatuses, because they not only provide good quality images with little power but also are very thin. In general, an LCD includes a liquid crystal panel and a backlight module for illuminating the liquid crystal panel.
In an active matrix liquid crystal display (AM-LCD) system, the character curve in FIG. 5 which shows the transmittance of the liquid crystal versus the applied gamma driving voltage in an actual AM-LCD, is a non-linear curve, not linear curve. In operation, the liquid crystal display changes the optical transmittance of the liquid crystal molecular through changing the applied driving voltage between the upper and the lower substrates, for displaying image. The applied driving voltage is named gamma voltage.
FIG. 6 is a character curve, which shows the transmittance of the liquid crystal versus the gray level. The character curve is named gamma curve. The relationship between the transmittance of liquid crystal and the gray level maintains the following function:T=Tmax*(G/Gmax)r wherein T represents transmittance of liquid crystal; Tmax Represents the maximal transmittance of liquid crystal; G represents gray level; Gmax represents the maximal gray level corresponding to the maximal transmittance of liquid crystal; r represents gamma value. In FIG. 6, the gamma value of the gamma curve equals to 1.0. When the relationship between the transmittance of liquid crystal and the gray level maintains the gamma curve of FIG. 6, the liquid crystal display has an ideal vision effect for human eyes. However, actually, the transmittance of liquid crystal and the gray level has a non-linear relationship. Thus, a special circuit for a liquid crystal display is needed to output corresponding gamma voltage to make the relationship between the transmittance of liquid crystal and the gray level maintains the gamma curve of FIG. 6, i.e. linear relationship.
Referring to FIG. 7, a typical gamma voltage output circuit is shown. The gamma voltage output circuit 1 is capable of outputting gamma voltage signals to display gray scale images with sixty-four levels. That is, the gamma voltage output circuit 1 can output sixty-four gamma voltages V1˜V64.
The gamma voltage output circuit 1 includes: a resistor string 11 connected between an analog electrical source (AVDD) and ground. The resistor string 11 includes sixty-five resistors R0˜R64 connected in series.
However, movable or portable display are usually operated under different external environment, such as cloudy day, sun day, or night, et. Under different external environment, the display images produce different color bias if only single gamma curve is used in the movable or portal display. That is the transmittance corresponding to the gray level cannot be properly displayed under different external environments. Thus, different gamma curves are needed for different external environments. Referring to FIG. 8, three different gamma curves are shown corresponding to three different external environments, which respectively represent gamma values of 1.0, 2.0 and 3.0.
FIG. 9 shows another typical gamma voltage output circuit which can provide three gamma voltages. The gamma voltage output circuit 2 includes a first resistance string 21, a second resistance string 22 and a third resistance string 23, respectively connecting in series between a power supply AVDD and ground. The first resistor string 21 has sixty-five resistors R0_1˜R64_1 and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V1_1˜V64_1. The second resistor string 22 has sixty-five resistors R0_2˜R64_2 and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V1_2˜V64_2. The third resistor string 23 has sixty-five resistors R0_3˜R64_3 and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V1_3˜V64_3. By adjusting the resistance value of each resistor, three gamma curves shown on FIG. 8 can be attained.
When three gamma curves is needed, the gamma voltage output circuit 2 has thrice the number of the resistors of the gamma voltage output circuit 2. However, when eight or ten or more gamma curves are needed, the number of the resisors of the gamma voltage output circuit can be enormous. For designing or manufacturing an integrated circuit (IC), more resisotrs, more cost.
In the gamma voltage output circuit 1, the voltage output from the analog electrical source is distributed to the resistors R0˜R14 of the resistor string 11, and the capacitors have a function of wave filtering. Each operational amplifier 12 improves the capability of equipping loads. The gamma voltage output from the output port of each operational amplifier 12 is equal to the voltage signal inputted into the non-inverting input port of the same operational amplifier 12. Thus, each gamma voltage can be calculated according to the following equations:V1=AVDD*(R1+R2+ . . . +R14)/(R0+R1+R2+ . . . +R14)V2=AVDD*(R2+ . . . +R14)/(R0+R1+R2+ . . . +R14). . .V14=AVDD*R14/(R0+R1R2+ . . . +R14)
In order to increase the precision of the resistors R0˜R14, the configuration of the resistor string 11 can usually be varied. Referring to FIG. 4, the resistors R01 and R02 are connected in parallel, and a resistance of the parallel connected resistors R01 and R02 is equal to that of the resistor R0. The resistors R11 and R12 are connected in parallel, and a resistance of the parallel connected resistors R11 and R12 is equal to that of the resistor R1. In other words, each pair of resistors Rm1 and Rm2 are connected in parallel, and a resistance of the parallel connected resistors Rm1 and Rm2 is equal to that of the resistor Rm (0≦m≦14). Thus the resistance of the resistors R0˜R14 can be suitably configured by controlling the resistances of the resistors Rm1˜Rm2.
When the gamma voltages need to be modulated, the resistances of the corresponding resistors need to be adjusted. For example, when the gamma voltage V2 needs to be modulated, then the resistance of the resistors R2 (R21 and R22) needs to be adjusted. However, according to the equations shown above, when the resistance of one of the resistors is varied, the value of other output gamma voltages also varies. That is, the gamma voltages output from the gamma voltage output circuit 1 affect one another, and cannot be adjusted individually.
Accordingly, what is needed is a gamma voltage output circuit that can overcome the above-described deficiencies.